Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt securities available for sale:
Due in one year or less	$ 3,490
Due after one year through five years	11,848
Due after five years through ten years	97,898
Due after ten years	30,000
Total available for sale (amortized cost)	143,236
Mortgage-backed securities available for sale	3,658,704
Investment securities available for sale, amortized cost	3,801,940	$ 4,621,027
Debt securities held to maturity:
Due in one year or less	371
Due after one year through five years	3,255
Due after ten years	2,665
Total available for held to maturity (amortized cost)	6,291
Mortgage-backed securities held to maturity	1,711,907
Amortized cost for held to maturity	1,718,198	1,748,989
Debt securities available for sale:
Due in one year or less	3,540
Due after one year through five years	12,440
Due after five years through ten years	98,672
Due after ten years	24,641
Total available for sale (fair value)	139,293
Mortgage-backed securities available for sale	3,820,490
Total	3,959,783	4,822,606
Debt securities held to maturity:
Due in one year or less	370
Due after one year through five years	3,257
Due after ten years	2,665
Total available for held to maturity (fair value)	6,292
Mortgage-backed securities held to maturity	1,778,722
Total	$ 1,785,014	$ 1,842,281